Method of manufacturing microlens array

ABSTRACT

A positive resist layer formed on a translucent substrate is subjected to exposure and development to form circular resist patterns disposed near each other at a predetermined distance. The exposure pattern has an opening of a plurality of circular ring shape surrounding the plurality of resist patterns. The opening may have a width equal to the predetermined distance. Since the width of the opening is constant over the whole outer periphery of each resist pattern, uniform exposure can be realized at all positions along the periphery to form a perfect circle shape. After a convex lens shape is given to each resist pattern by heating and reflow process, the convex lens shape is transferred to the substrate by dry etching process using the resist patterns as a mask to form a microlens array with convex lenses.

CROSS REFERENCE TO RELATED APPLICATION

This is a Divisional of U.S. patent application Ser. No. 10/768,024filed on Feb. 2, 2004, in the name of Toshihiro Nakajima, entitledMETHOD OF MANUFACTURING MICROLENS ARRAY, claiming priority of JapaneseApplication No. 2003-030908, dated Feb. 7, 2003 and the entiredisclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

A) Field of the Invention

The present invention relates to a microlens array manufacture methodwhich transfers lens shapes to a transparent substrate or underlyinglayer by etching using a resist pattern as a mask, and to a microlensarray produced.

B) Description of the Related Art

A microlens array having regularly aligned microlenses is used in aphotocoupler for an optical fiber array, an optical integrated circuit,a solid state imager, an optical system of an electronic copy machine, aliquid crystal display and the like. According to a conventionalmanufacture method for such a microlens array, circular positive resistpatterns are formed and changed to convex lens shapes by heating andreflow process, and thereafter the convex lens shapes of the positiveresist patterns are transferred to a substrate by a dry etching process(for example, refer to Japanese Patent Laid-open Publication No.HEI-7-174903).

With this method, as shown in FIG. 7, on one principal surface of aquartz substrate 1, a necessary number of circular positive resistpatterns 2 a to 2 d are disposed in line. Each resist pattern is spacedfrom other resist patterns so as not to contact each other. The regionsoutside the circular patterns 2 a to 2 d are exposed to leave un-exposedcircular patterns. Next, the resist patterns 2 a to 2 d are heated andreflowed to form convex lens shapes (spherical convex shapes) by asurface tension. Thereafter, by using the resist patterns 2 a to 2 d asa mask, the substrate is dry-etched to transfer the lens shapes of theresist patterns to the principal surface of the substrate. In thismanner, a microlens array can be formed on the substrate 1, havingconvex lenses of circular plan shapes linearly aligned in correspondencewith the resist patterns 2 a to 2 d.

Generally, a larger exposure energy is required at a narrower exposureline (or area) width, when resist patterns are formed by subjecting apositive resist layer to exposure and development process. If theexposure area is narrower, a higher exposure energy is needed totransfer the mask image at a high fidelity. In the example shown in FIG.7, a larger exposure energy is required at a position nearer to thecenter line Lc intersecting the centers of the resist patterns 2 a to 2d. An exposure process is performed at such an exposure energy as properresist patterns are formed on the center line Lc. With this setting,although proper exposure is performed on the center line Lc along an Xdirection, the exposure line (or area) width becomes broader at aposition remoter from the center line Lc in a Y direction perpendicularto the X direction, resulting in excessive exposure. Proper resistpatterns in conformity with the reticle patterns cannot be obtained.

FIG. 8 shows cross-section of a convex lens La formed on the substrate 1by transferring the resist pattern 2 a, the convex lens La correspondingto the resist pattern 2 a formed by the above-described method. Lx andLy indicate the cross section of the lens La along the X and Ydirections. As described above, although the lens pattern 2 a issubjected to proper exposure on the center line Lc along the Xdirection, excessive exposure is performed along the Y direction awayfrom the center line Lc. The lens La is not a perfect circle, and theradius of curvature in the X direction becomes larger than that in the Ydirection. Thus, a focal point Py in the Y direction becomes nearer tothe substrate than a focal point Px in the X direction. A focal lengthdifference (astigmatism) between the sagittal direction (X direction)and the meridional direction (Y direction) becomes large so that theoptical characteristics are degraded.

SUMMARY OF THE INVENTION

An object of this invention is to provide a micro lens array manufacturemethod capable of preventing deformation of a lens plan shape.

According to one aspect of the present invention, there is provided amethod of manufacturing a microlens array comprising the steps of:forming a positive resist layer on one principal surface of atranslucent substrate; subjecting the positive resist layer to exposureprocess and development process to form a plurality of circular resistpatterns disposed near each other at a predetermined distance, in whichthe exposure process is performed by using an exposure pattern allowingto form an opening of a ring shape surrounding the plurality of circularresist patterns and thereafter the development process is performed;giving each of the plurality of resist patterns a convex lens shape byheating and reflow process; and transferring the convex lens shape ofeach of the plurality of convex lenses to the principal surface of thetranslucent substrate by dry etching process to form a microlens arrayhaving circular convex lenses corresponding to the plurality of resistpatterns on the principal surface of the translucent substrate.

An exposure line width along an outer periphery of each resist patternis almost constant. Almost uniform exposure is possible along the outerperiphery of each resist pattern at an exposure energy determined by theconstant exposure line width. Deformation becomes small under theuniformed conditions. A better circle pattern can be obtained for eachresist pattern.

Since a convex shape is given to each resist pattern, it is possible toform a better circle convex lens on the substrate surface. A convex lenshaving the equal radius of curvature in all directions has an equalfocal length in all directions.

A difference between focal lengths in the sagittal and meridionaldirections can be reduced considerably so that a microlens array havinggood optical characteristics can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 2, 3 and 4 are cross sectional views illustrating mainprocesses of a microlens array manufacture method according to anembodiment of the invention.

FIG. 1B is a schematic diagram showing a reducing projection stepperapparatus.

FIG. 5 is a plan view of resist patterns at the process shown in FIG. 2.

FIG. 6 is a plan view of resist patterns to be used when atwo-dimensional microlens array is manufactured.

FIG. 7 is a plan view of resist patterns formed by a conventionalmicrolens array manufacture method.

FIG. 8 is a cross sectional view of a lens manufactured by using theresist pattern shown in FIG. 7.

FIG. 9A is a partial enlarged view of FIG. 5.

FIG. 9B is a partial plan view showing a modification of resistpatterns.

FIGS. 10A and 10B are cross sectional views showing a modification of alens shape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A, 2, 3 and 4 illustrate a microlens array manufacture methodaccording to an embodiment of the invention. Processes (1), (2), (3) and(4) corresponding to FIGS. 1A, 2, 3 and 4 will be described in thisorder. FIGS. 1A, 2, 3 and 4 correspond to cross sectional views takenalong line R–R′ shown in FIG. 5.

(1) On one principal surface of a quartz substrate 10, a positive resistlayer R₀ is formed by spin coating or the like. As shown in FIG. 1A, thepositive resist layer R₀ is selectively exposed by using a 1:1 exposureapparatus such as a proximity aligner and an exposure mask 12.

The exposure mask 12 has, as shown in FIG. 5, a translucent area Tcorresponding to an opening N having the shape of a chain of rings. Apartial area T₁ of the translucent area T corresponds to an opening N₁between positive resist patterns R₁ and R₂, and another partial area T₂of the translucent area T corresponds to an opening N₂ between positiveresist patterns R₂ and R₃.

Other types of aligners may be used in place of the 1:1 aligner. FIG. 1Bis a schematic diagram showing a reduction projection aligner. If areduction projection aligner is used, an exposure mask 12× is a maskscaled up from the exposure mask 12 shown in FIG. 1A by a fivefold,tenfold or the like, and has an analogous translucent area T and a lightshielding area S. The translucent area T has the shape of a chain ofrings similar to that shown in FIG. 5. A projection lens PL reduces thesize of patterns on the exposure mask 12× by one fifth, one tenth or thelike and projects the patterns on a substrate 10.

The positive resist layer R₀ is exposed by an exposure patterncorresponding to the translucent area T of the exposure mask 12. Anexposed area E of the positive resist layer R₀ corresponds to thetranslucent area T, and exposed areas E₁ and E₂ correspond to thepartial areas T₁ and T₂ of the translucent area T. The exposed area Ehaving the shape of a chain of rings and corresponding to the opening Nshown in FIG. 5 surrounds the partial areas (patterns) R₁ to R₄ in thering shape.

(2) The exposed positive resist layer R₀ is developed to remove theresist in the exposed areas E. As shown in FIGS. 2 and 5, resistpatterns are therefore formed which are constituted of the positiveresist areas R₀ to R₄. The resist patterns have the opening N having theshape of a chain of rings and corresponding to the exposed area E.

The resist patterns R₁ to R₄ are disposed near each other at apredetermined distance D. The distances in proximate areas between theresist patterns R₁ and R₂, between the resist patterns R₂ and R₃, andbetween the resist patterns R₃ and R₄ are all D. The opening N surroundsthe resist patterns R₁ to R₄ in a ring shape and has a width W equal tothe distance D. For example, assuming that the length A and width B ofthe rectangular substrate 10 are 3 mm and 1.5 mm, D and W can be set to8 μm and the diameter of each of the resist patterns R₁ to R₄ can be setto 496 μm.

In the exposure process shown in FIG. 1A, an exposure pattern formingthe opening N having the shape of a chain of rings is used so that anexposure line width along the outer periphery of each of the resistpatterns R₁ to R₄ becomes constant or D. By setting an exposure energydetermined from this constant exposure line width, uniform (notexcessive nor insufficient) exposure can be performed at any positionalong the outer periphery of each resist pattern. Representing thedirection of a center line connecting the centers of the resist patternsR₁ to R₄ by an X direction and the direction perpendicular to the Xdirection by a Y direction, each of the resist patterns R₁ to R₄ is aperfect circle having the same diameter in both the X and Y directions.

(3) The resist patterns R₀ to R₄ are heated and reflowed to make theresist patterns R₁ to R₄ have a convex lens shape. All the resistpatterns R₁ to R₄ have a surface shape in conformity with the surfacetension (and gravitation force). The lens top surface is slightly higherthan the upper level of the resist pattern R₀.

(4) By using the resist patterns R₀ to R₄ as a mask and a dry etchingprocess using gas such as CF₄ and CHF₃, the convex lens shapes of theresist patterns R₁ to R₄ are transferred to one principal surface of thesubstrate 10. In this manner, a microlens array is formed which has fourlinearly disposed convex lenses corresponding to the resist patterns R₁to R₄ on one principal surface of the substrate 10. FIG. 4 shows threeconvex lenses L₁₁ to L₁₃ among the four convex lenses. Each convex lensformed in the above manner has a perfect circle corresponding to aperfect circle pattern of each of the resist patterns R₁ to R₄, has thesame radius of curvature in both the X and Y direction and has the samefocal length in both the sagittal direction and meridional direction. Alinear microlens array having the good optical characteristics cantherefore be obtained.

FIG. 6 is a diagram showing resist patterns to be used for manufacturinga two-dimensional microlens array. A two-dimensional microlens array canbe manufactured easily by changing only the size of the quartz substrate10 and resist patterns used by the linear microlens array manufacturemethod described with reference to FIGS. 1A to 5.

The substrate 10 may by a quartz substrate of a square shape having aside K of 6 mm. On one principal surface of the substrate 10, a positiveresist layer R₁₀ is formed in the manner similar to that described withreference to FIG. 1A. The positive resist layer R₁₀ is subjected to anexposure and development process to form sixteen resist patterns R₁₁ toR₁₄, R₂₁ to R₂₄, R₃₁ to R₃₄, and R₄₁ to R₄₄ in a two-dimensional layout(matrix layout). All the resist patterns R₁₁ to R₄₄ have a perfectcircle.

The resist patterns R₁₁ to R₁₄ are disposed near each other at apredetermined distance D along the X direction. Similar to the resistpatterns R₁₁ to R₁₄, the resist patterns R₂₁ to R₂₄, R₃₁ to R₃₄, and R₄₁to R₄₄ are also disposed near each other at the predetermined distanceD. The resist patterns R₁₁ to R₄₁ are disposed near each other at thepredetermined distance D along the Y direction perpendicular to the Xdirection. Similar to the resist patterns R₁₁ to R₄₄, the resistpatterns R₁₂ to R₄₂, R₁₃ to R₄₃, and R₁₄ to R₄₄ are also disposed neareach other at the predetermined distance D. An exposure process isperformed by an exposure pattern having an opening N′ which surroundsthe resist patterns R₁₁ to R₄₄ in a ring shape and has a width W equalto the distance D, and then a development process is performed. In thismanner, the plan shapes of all the resist patterns R₁₁ to R₄₄ can bemade perfectly circular. For example, D and W can be set to 10 μm andthe diameter of each of the resist patterns R₁₁ to R₄₄ can be set to 990μm.

The resist patterns R₁₁ to R₄₄ are heated and reflowed to make theresist patterns R₁₁ to R₄₄ have a convex lens shape. By using dryetching process, the convex lens shapes of the resist patterns R₁₁ toR₄₄ are transferred to one principal surface of the substrate 10. Inthis manner, a microlens array is formed which has sixteentwo-dimensionally disposed convex lenses corresponding to the resistpatterns R₁₁ to R₄₄ on one principal surface of the substrate 10. Eachconvex lens formed in the above manner has a perfect circlecorresponding to a perfect circle pattern of each of the resist patternsR₁₁ to R₄₄, has the same radius of curvature in both the X and Ydirection and has the same focal length in both the sagittal directionand meridional direction. A two-dimensional microlens array having thegood optical characteristics can therefore be obtained.

In the above-described embodiment, the distance D between the lenses isequal to the exposure ring width W.

FIG. 9A is an enlarged view of a connection area between rings. Althoughthe width of the exposure area at the most proximate position of lensesis D, the exposure width broadens to about 2D at the upper and lowerpositions of the proximate area. Since a change in the exposure areawidth is a twofold at the most, a high exposure precision can berealized.

FIG. 9B is an enlarged view of rings according to a modification of theembodiment which can improve the exposure precision. Exposure areasE_(i) and E_(i+1) surrounding the rings are circular rings separatedfrom each other. Lens areas L₁ and L_(i+1) are surrounded by thecircular rings E₁ and E_(i+1) having the equal width so that theexposure conditions can be made more uniform.

In FIGS. 3 and 4, the resist patterns flow slightly during the reflowprocess and the ring openings are drawn distinguished. The ring openingsmay be left.

FIG. 10A shows the state that the opening is left around the lens resistpatterns and a groove G in the shape of a chain of rings is left isaround the finished lenses.

FIG. 10B shows the state that the separated circular ring openings shownin FIG. 9B are left around the microlens array as grooves G′.

The substrate may either a bulk substrate or a laminated substratehaving an upper layer for forming a lens array.

The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It will be apparent to those skilled in the art that othervarious modifications, improvements, combinations, and the like can bemade.

1. A method of manufacturing a microlens array comprising the steps of:forming a positive resist layer on one principal surface of atranslucent substrate; subjecting said positive resist layer to exposureprocess and development process to form a plurality of circular resistlayers disposed near each other at a predetermined distance, in whichthe exposure process is performed by using an exposure pattern allowingto form an opening of a circular ring shape surrounding said pluralityof resist layers and thereafter the development process is performed;giving each of said plurality of resist layers a convex lens shape byheating and reflow process; and transferring the convex lens shape ofeach of said plurality of resist layers to the principal surface of saidtranslucent substrate by dry etching process to form a microlens arrayhaving circular convex lenses corresponding to said plurality of resistlayers on the principal surface of said translucent substrate, whereinsaid opening is formed integrally in the shape of a chain of rings. 2.The method according to claim 1, wherein said opening is formedseparately for each of said plurality of circular resist layers.
 3. Themethod according to claim 1, wherein said opening has a width equal tothe predetermined distance.